1. Field of the Invention
The invention relates to a device for holding measurement instruments, in particular interferometers, formed from a plurality of structure elements connected to one another and made from at least one material, the at least one material having a very low coefficient of thermal expansion α, in particular being a glass-ceramic or a metal alloy.
Moreover, the present invention also relates to a device for the stress-free connection of at least two components.
2. Description of the Related Art
In the case of optical systems which are intended to be suitable for generating optical images with a very high level of accuracy, for example optical systems for lithography, the individual optical elements have to be positioned as accurately as possible in all six degrees of freedom in terms of their orientation.
Optical systems of this type often include sensors which are able to monitor the position of the optical elements and carry out path length measurements or measurements relating to the change in path length.
If optical systems of this type arc used in practice, loads on the optical system itself may arise on account of temperature fluctuations which may occur through environmental influences. These temperature fluctuations may have a serious effect on the results, in particular in the case of objectives used in semiconductor lithography; to achieve a very high level of measurement accuracy, the influence of thermal expansion should be minimized.
DE 101 34 387.6, which was not published before the priority date of the present application, describes an objective, in particular a semiconductor lithography, which has two separate structures. A load structure is intended to hold optical elements, in particular mirrors, while a further structure is designed as a measurement or reference frame. The load structure in this case bears the entire load of the optical element and the reaction forces which occur in the event of corresponding movements. The measurement structure, on the other hand, bears only its own weight, so that a highly stable measurement structure can be achieved.
A measurement frame of this type can be produced from materials which are known in the prior art with a very low thermal expansion, such as for example ZERODUR®, ULE or the like. However, large measurement frames have to be assembled and connected from a plurality of structure parts made from materials of this type. Although the materials used have a low thermal expansion, they are not suitable for producing connecting elements required for assembly, on account of their mechanical properties. In the region of the connections between the individual structure elements, the materials used there for the connecting elements means that the thermal expansions will be very much greater. The geometric dimensions of the overall measurement frame will therefore change when temperature fluctuations occur to a sufficient extent for the required degree of accuracy in the nanometer range no longer to be achieved.
Furthermore, in connection with the prior art reference is also made to U.S. Pat. No. 6,227,754 B1, U.S. Pat. No. 6,111,691 A and U.S. Pat. No. 6,099,193 A.